The main material to be dried in the chamber disclosed by the present invention is lumber. The following description is made with lumber as the example.
In the past, green lumber had been naturally seasoned in the open air in almost all cases. However, artificial drying methods have recently been made more widely known. There are many methods for artificially drying lumber to the desired degree of moisture content. However, the most basic method for said artificial drying is to heap or place lumber in a pile in a drying tank, and to expose said lumber to hot air for drying for a certain given time.
In the prior art, lumber to be dried is accommodated in a pressure-resisting tank whose pressure is raised to 1 to 18 times atmospheric pressure, and is simultaneously exposed to a vapor jet for 1 to 3 hours. The inside pressure of said tank is reduced to 600 mmHg, thereby reducing moisture. The lumber so treated is placed in a pile in a sealed chamber and hot air having a temperature of approx. 45.degree. C. is applied by a blower to said lumber in an airflow in the chamber. Said hot air is circulated, thereby reducing the moisture content of the lumber to less than 10%. This drying method has been disclosed in Japanese Pat. Publication No. 33001/1973.
Recently, a vacuum drying method, by which a sealable chamber is prepared and lumber is dried with the inside pressure of said chamber reduced, has been proposed, thereby greatly shortening the drying time. However, the control of this method such as the temperature, the humidity and moisture and/or pressure reducing is not automatic but is by human operation and depending upon skill from long experience.
Therefore, the control of temperature, humidity and/or pressure reduction in this method is not always suitable for drying lumber, thereby causing such defects as surface hardening, drying unevenness, and surface/inside cracks or splitting.